Petroleum exists as a liquid in an oil field covered with solid rocks. Natural gas exists as a gas in a gas field under the ground. In extracting petroleum, a boring machine is used to drill an extraction hole in the ground, so that petroleum is extracted through the extraction hole. In a new oil field, petroleum gushes itself by internal pressure of the oil field. On the other hand, in an old oil field, a pump is used to suck up petroleum. In this case, it is impossible to extract all petroleum in the oil field. Thus, high-pressure water or the like is supplied into the oil field, so that petroleum can be extracted from the oil field.
When extracting petroleum and natural gas, they are accompanied by CO2 (carbon dioxide) or H2S (hydrogen sulfide). Because CO2 can be the cause of global warming, emission of CO2 should be as low as possible. However, it is conventional to release CO2 into the atmosphere as it is without any processing. On the other hand, since H2S is a harmful gas, it has been customary to capture H2S by desulfurization equipment and bury the captured H2S in the ground.
In recent years, it has been a trend to recover CO2 or H2S, compress them by a compressor, and cool them to produce a liquid or supercritical fluid thereof. The liquid or supercritical fluid produced is further compressed by a high-pressure pump and injected under the rocks, so that CO2 or H2S is returned to the ground. In order to fix CO2 and H2S in a supercritical state stably in the ground (e.g., under the rocks), high pressure is required. A multistage high-pressure pump is most preferable as a pressure source in view of its volume and its shaft seal structure. This method enables stable extraction of petroleum and natural gas and can return CO2 or H2S under the rocks. Therefore, not only can this method allow efficient use of CO2 and H2S, it can also be an environmentally-friendly method.